PV system design – Array sizing
Array sizing is important aspect in PV system design .Generally people do array sizing using PVsyst or any other software. However we have little understanding on different aspects/variables they should consider while array sizing .In a typical solar PV plant each array is combination of number of modules first connected in series to achieve a desired voltage and then connected in parallel to allow system to produce desired current. Designing a PV array has various criteria and constraints which include the site layout conditions, local shading issues, linear shading issues, inverter voltage and current ratings, site solar irradiation etc.
Site constraints lay a important role for example ,there may be a linear shading or inter row shading which will restrict your array size to be of a smaller length or smaller width .It can be explained through following examples, the typical 280 W (35V,8A) PV module is of 1.6 m length and about 1 m width. If we connect 20 modules in series the total length of the array will be about 24 m.Following figure show different combination to have an array with 20 modules arranged in a single string. If this modules are staged up each other the length of array can be reduced to 20 to 10 m .However vertical stack up of modules each other will lead to inter-row shading and the distance between the two array needs to be increased .In some of the design EPC contractors go for 3 or 4 stacking of PV modules which increases the array width significantly and becomes difficult for cleaning the modules ,which are in the upper most portion of the array layout.
Array sizing is also dictated by inverter capacity and input voltage. For example a typical PV with 500KW ABB inverter has input voltage 400-800V, if we need to connect a 280 W (35V 8A)module. It is recommended that a single array size should have 20 module in series , which will lead to 700 voltage .The array sizing is also affected by the local temperature conditions, If the highest temperature of the site goes to 50 °C. Its important to take in to consideration, the local site temperature and accordingly the array sizing is done. For example in the above case , if the modules voltage temperature coefficient is -0.14 V/°C. The string voltage at 50°C will be of the order of 630 V. It may be noted that modules operating temperature is the surface temperature, not the ambient temperature and module surface temp typically 20 to 30°C higher , hence while the ambient temperature is 25°C ,the module operating temperature is 50 °C and array voltage even at 25°C the array voltage is 630V.
For a particular site of temperature variation from (0-50) °C, for a string of 20 modules of 280 W (35 V , 8 A),the inverter voltage range has to be in meet above calculated voltage levels for efficient operation of the plant.
In state like Rajasthan the ambient temperature reaches as high as 50°C and modules surface temperature reach of the order of 75 °C and accordingly the string voltage is of the order 560 V. It is equally important that we consider the minimum temperature of the site conditions while doing the array layout .During winters the temperature go as low as 0 °C and the module surface temp is also reaching to °C and hence the voltage of the string reach 770 V. It may be noted that the temperature has the greater impact on voltage ,while the irradiance has greater impact on current .Hence while selecting array size maximum irradiance and minimum irradiance will dictate cable size for interconnecting the modules in the string.
Dr. Sanjay vashishtha & Rishikesh Muthyal